engine misfire troubleshooting
 

Hi all,

My 912E is giving me fits. It will start and idle fine. When I drive after it has sat for awhile, it runs fine. When it gets hot, it develops what seems like a miss, and loses a lot of power. The exhaust note also seems to get louder.

Car has:

40 IDF carbs
Permatune
breaker points
050 distributor new last year, with new cap, points, rotor, condensor, etc.

Here's what I've done recently, in chasing down other problems.

New plugs
Checked wires and ends with an ohm meter for conductivity
engine timed
valves adjusted
carbs completely disassembled and cleaned.
carbs reinstalled and balanced with new gaskets-no known vacuum leaks.

Any ideas? What would cause ignition to crap out when hot? Is this symptomatic of a bad condensor? Coil?

Also, is anyone running Nology wires? I've been thinking about buying a set and now seems like a good time. Any thoughts on Nology wires?

Thanks,

David

Check for exhaust leaks.

Check the points--make sure the gap is good, that the points themselves are in good shape, and that the dwell doesn't change with RPM. The points "shouldn't" go bad in a year, but...

Try hooking the ignition up without the Permatune. I know they say they only fail completely when they do fail, but I know one person who has had one or two fail intermittently.

Not sure what else offhand...

--DD

I had a similar problem just a month ago. Mine is FI but that had nothing to do with it.

I replaced the points with a Pertronix Ignitor, new coil, new cap and rotor, new wires, and new plugs. Still had the miss. Bought 2nd set of new wires and miss went away. My new wires were bad out of the box. You might want to try that.

David Pritchard

Check for overheating coil (internal breakdown under heat, possibly resulting in short).

Hi Dave,

I agree with Dave. Check for exhaust leaks. Make sure the intake manifold nuts are on tight.

Nology wires? Why? Check out the Permatune "Plasma Drive" C/D units. Now that I can see will work. Check em out at www.perma-tune.com

thanks for the tips guys. I will be replacing the wires tonight. Where is a good source for the appropriate spark plug connectors?

Dave Darling, does PP stock 912E connectors? Are they the same part as for a 914 2.0? I've read that they are different for different 914 models, I want to make sure there isn't some oddball 912E connector?

I don't think I have any exhaust or vacuum leaks. I will recheck, but it's all new this spring. Also, the intake manifold/head gaskets are brand new (the thick, "phenolic" ones).

Re: Nology-it sounded like a good product. But after talking with their sales rep, who hounded me with about six emails and five phone calls in two days, I have to say I am totally unimpressed with them. The rep actually insulted me and my mechanic, over the phone, before I got through explaining the problem and the circumstances. I actually hung up on him he was such an idiot.

Has anyone ever used a Permatune coil?


David

You know, based on the symptoms, you might check your valve clearance again. Maybe something got mixed up or not set correctly the last time, and after things heat up, it's causing your valve clearance to be out of whack. That may be attributable to the changing exhaust note also.

Everything but the FI and exhaust on a 912E motor is identical to that used on the 2.0 914 motor. That includes the plugs and plug wires.

--DD

DD,

do you recommend any specific PP part number? I'd like a complete set of everything except the wires (I have a relocated coil and will have to cut my wires from bulk stock).

David

Okay guys, an update:

a new set of wires have been installed.
carbs off, cleaned again, no junk found there.
fuel flow checked out okay.
new fuel filter anyway.
float levels okay in the carbs.
leak down (4% across the board) and compression check (130,130,120,110) completed.

The car STILL won't run right. I've even had the same symptoms if I purposefully set the timing wrong (not enough advance) just to convince myself it's not a timing issue.

Again, here's what happens: it will idle okay and I can balance the carbs fine with the unisyn. But, under any load or acceleration, it develops a nasty miss and has zero power. And, the exhaust pops when this symptom is occurring.

Any more ideas or thoughts?

Thanks, as always.

Maybe check the carb balance at 3000RPM? Could be that the idle balance is OK but the linkage is not adjusted right so when you come off idle one carb is opening much more than the other.

Try bypassing the Permatune and run as just a plain points system (and try new points) to test.

Again, check the coil for thermal breakdown. Get a used one or get a new one and replace yours - see if that helps. Just because you have spark (what color is it?) doesn't mean the coil is not breaking down when the temp rises.

Poor acceleration is generally - not enough dwell, retarded timing or no fuel flow - BUT if the coil is breaking down, it won't fire the plugs.

Even tho you have a "new" distributor, how much movement in the shaft is there (it is a year old)? Are the advance weights or mechanisms sticking (no mechanical advance).

Which rotor do you have - normal or speed limiting? If speed limiting try changing back to the normal one.

NOTE: Ignore this post, I see you bought a new coil. I assume the leads to the coil are hooked up correctly.

To all of the guys who have taken the time to send their ideas:

First off, thanks for the ideas. I appreciate it.

Second, I swapped coils back and forth, no difference. And, I can see spark jumping if I ground the plugs on the block or the body or wherever. No problem there.

I will try the different rotor, although something tells me that's not the problem.

Perhaps I will pull the distributor and have my mechanic eliminate that possibity. He has a distributor machine and recurves them for people all the time.

Today's idea is to install a fuel pressure regulator. The fuel pressure is high, like 6 psi. However, the car ran fine last year and the pump is the same. I measure this before the new fuel filter went in, so it was high then also. In short, it ran fine last year and the pressure is now no higher than it was before. And, my symptoms are more like fuel starvation than too much pressure. But, it's something I want to eliminate as a variable anyway.

I'm still concerned I have clogged up circuits within the carb body. Any way to plumb them out? I've had the carbs apart several times (completely apart) and let them both sit in a vat of nasty parts cleaner, so I was hoping that cleaned them out sufficiently. But, then again?

I'll keep you all posted.

Thanks,

A year ago, your float needles were new?

On our race cars, the max fuel pressure I ran was 5 psig. At that level of pressure and the bouncing of the float in the bowl, I would reset the float level damn near every race as the float levers would bend.

For a street motor, 3.5 psi is great. It doesn't over power the float needles.

For cleaning carb passages and jets, I buy a can of carb or brake cleaner, a can of compressed smog (at least here in LA) and a gasket set (sometimes). Take 'em apart, spray cleaner with the tube nozzle thru all the passages, followed by the compressed air with tube nozzle.

It's funny that the computer compressed air is more expensive than the cleaner.:confused:

Ron,

Good advice. Ironically, that's what I spent Saturday doing and the car seems to run better-although I used WD-40 and the red straw taped to the bottle instead of computer cleaner. That's a good idea, though.

I also went down one main jet size. CB Performance recomended 130 mains when I called and explained my problem. It didn't seem to help at all. I left them in per their advice. Just because I was frustrated, I went back to 115 mains. Then it ran better.

It's wierd, I autocrossed it Sunday because it seemed to run better. On the first and last runs, it ran great. On the two in the middle, it ran like crap. After the autocross, it ran fine and has all day today as well. I am certain I have multiple problems occurring and that's what is making things difficult.

As a matter of course, I'm going to buy new floats and needles/seats. Is there a better source that you all know about beyond CB Performance?

Thanks,

David

For Weber parts I use Pierce Manifolds and I usually buy several rebuild kits at a time and make sure they have new needle and seats. I set the float to 12 and 24 mm which means the upper is just a little low which prevents fuel splashing on hard cornering. Additionally I agree with Ron on the fuel pressure and you could also check the accelerator pumps to make sure they are set the same and have the correct stroke. Finally a couple more items: are you getting any fuel leaking from the accelerator jet when the engine is idling and how loose are the throttle shafts, especially where they go between the barrels? You could also make sure the plug wires are mounted to an insulator and not flopping around or shorting to a ground on the engine. Good luck.

David, I had a similar problem with my 914. The heat generated from the engine boiled the fuel in the carbs and caused it to run lean and miss at high rpm (5000+). I installed the thick plastic insulated gaskets between the head and intake and solved the problem. The lean condition caused more heat to be generated and the exhaust to sound different. Good luck. Duane

Duane:

I have always used those plates (I believe they are called phenolic insulators or something like that) to control the heat transfer to the intake manifolds. That is a good thought, and thanks for taking the time to post.

John,

When you speak of the float settings, are you talking about IDFs? I thought the measurement was a static 10mm? Could you describe how you measure and adjust your floats? I've never fooled with them much, but if I knew what I was doing I'd try different heights to see if it had any effect.

The plug wires are tied up to avoid any grounding issues and they are also brandy new.

Thanks for the tip on Pierce.